Pressure sensors and especially pressure difference sensors are usually specified for a measuring range, in which they provide a pressure-dependent, measured value, wherein, in the ideal case, they should withstand significant overloading beyond the specified measuring range without damage to the sensor. This is the case all the more so for pressure difference sensors, as, in this case, the static pressures, whose difference is to be ascertained, can by all means exceed the measuring range of the difference to be ascertained by a factor of, for example, a thousand, so that in the case of a malfunction, when only one side of the measuring membrane is exposed to a pressure difference with a static pressure, a thousand-fold overload can easily arise. Measuring membranes of pressure difference sensors must be protected against this.
For this, an approach exists for the provision of overload membranes, which are connected in parallel to the measuring membrane of the pressure sensor in a hydraulic measuring mechanism. These overload membranes have a larger hydraulic capacity than the measuring membrane of the pressure difference sensor. The two static pressures, whose difference is to be ascertained, are, in the case of such pressure difference sensors, usually introduced into the measuring mechanism via pressure transfer means having process membranes or separating membranes, which are in each case arranged over a membrane bed. In the case of a one-sided overload, the deflection of the overload membrane accommodates the volume under the separating membrane exposed to the overload, until the latter bears down on the membrane bed, whereby a further pressure rise on the measuring membrane is prevented.
Pressure difference measuring devices with such an overload membrane are available commercially from the assignee under the marks Deltabar S and Deltabar M.
Another approach is based on the provision of a membrane bed for the measuring membrane, on which the measuring membrane bears down in the case of a one-sided overload, whereby the measuring membrane is supported and is protected from damage.
Both approaches have their specific weaknesses, for an overload membrane leads, as a rule, to a greater hydraulic volume, which, in the case of a given pressure, leads to greater forces and therewith to a more complex mechanical construction.
A support for the measuring membrane on a membrane bed, which, for example, has a contour, which corresponds to the deflection curve the measuring membrane, is, for example, only compatible with the established capacitive measuring transducers under certain conditions, since this support usually has, as opposed to a full surface electrode on the measuring membrane, an annular reference electrode, which surrounds a circular measuring electrode, wherein the measuring electrode and the reference electrode have in the high position of the measuring membrane the same capacitance with respect to the electrode of the measuring membrane.
In such case, the measuring electrode and the reference electrode especially have the same distance from the measuring membrane. In this respect, the reference capacitance between the reference electrode and the electrode of the measuring membrane changes to a significantly smaller degree than the pressure-dependent capacitance between the measuring electrode and the electrode of the measuring membrane when the measuring membrane is deflected in a pressure-dependent manner. This assumption is no longer valid when the annular reference electrode and the measuring electrode are arranged on a membrane bed, which has a contour, which is predetermined by the deflection curve of the measuring membrane.
Additionally, the danger exists that a measuring membrane supported on the membrane bed remains clinging to the membrane bed after the overload goes away.